An asynchronous motor equipped with magnetic bearings to magnetically suspend the entire rotor assembly, such as those used in turbomolecular pumps, generally provide an electronic circuit for motor braking. The circuit generally applies a braking action to the motor, which in turn supplies back an electric current generated from the EMF of the motor's windings, to a feeding circuit comprised of switching transistors when the driving frequency of the motor decreases.
The electric current generated during the braking phase is usually dissipated in a resistor, which comes on-line as part of the circuit when the brake is actuated.
In such high speed turbomolecular pumps a dedicated circuit generally provided to supply current to the magnetic suspensions and deenergize them when the motor is not in operation. For example, see Hablanian, M. H. "Handbook of Vacuum Technology", Marcel Dekker, Ink, 1990.
In normal operation an AC power source provides energy to the pump motor via a feeding circuit comprised of a main capacitor and an inverter. However in the case of a power source failure, the magnetic suspensions are abruptly deenergized and the rotor assembly of the asynchronous motor returns to its rest position atop the mechanical bearings, normally used only for start-up and final resting support, at the maximum rotational speed which is typically in the order of 36,000 rpm, thus causing a failure of said bearings.
In order to prevent failure of the mechanical bearings, suitable regeneration devices coupled to a switching circuitry can be installed which utilize a battery and an inverter combination, to ensure feeding a current to the magnetic suspensions is maintained until the rotor of the asynchronous motor has stopped.
As an alternative, the bearings supporting the rotor can be made more resistant to mechanical stresses such as those imposed by the sudden and high rotational forces contemplated herein, in order to reduce the consequences of a sudden fall of the magnetic suspensions caused by a mains failure.
These measures, however, have the shortcoming of being complex and costly both in terms of their installment and their proper operation, particularity in view of the cost and limited life of batteries utilized in such devices.